Heat oven to 200C/180C fan/gas 6.
Roll out the pastry and use to line a
23cm loose-bottomed tart tin. Prick the
base with a fork, and line with baking
parchment and baking beans. Bake blind
for 12 mins, then remove the beans and
parchment. Return to the oven for a
further 5 mins until pale golden. Reduce
oven to 180C/160C fan/gas 4.
Meanwhile, make the filling. Melt the
butter in a large pan and, when foaming,
add the leeks, cover and cook on a low
heat for 20 mins until soft. Leave to cool.
Beat together the eggs, cream, milk,
mustard, Caerphilly and some seasoning.
Evenly cover the base of the pastry case
with the leeks, then pour in the egg
mixture. Bake for 25 mins.
To make the topping, combine all the
ingredients. Remove the tart from the
oven and sprinkle the topping evenly
over. Return to the oven and bake for
a further 15 mins until golden brown.